As one of the light exposure methods of photographic photosensitive materials, there has been known a image forming method of so-called scanner type, in which an original image is scanned and which a silver halide photographic material is light-exposed based on the obtained image signals to form a negative or positive image corresponding to the image on the original image.
As for a case where an image output from a scanner on a film is further printed directly on a printing plate without any reversal step, or a scanner light source having a soft beam profile, a light-sensitive material for scanners exhibiting ultra-high contrast property has been desired.
A large number of photosensitive materials having a photosensitive layer on a support for forming images upon imagewise exposure have been known. Among them, as a system for rendering preservation of environments and image forming means simplified, a technology for forming images by heat development is exemplified.
In recent years, reduction of the amount of waste processing solutions is strongly demanded in the field of photomechanical processes from the standpoint of environmental protection and space savings. To cope with this, techniques are needed in relation to photosensitive heat developable materials for use in photomechanical processes, which can be effectively exposed by a laser scanner or laser image setter and can form clear black images having high resolution and sharpness. Such heat developable photosensitive materials can provide to customers a heat development processing system, without use of solution-type processing chemicals, simpler and free from incurring environmental destruction.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Morgan and B. Shely, Imaging Processes and Materials, "Thermally Processed Silver Systems", 8th ed., page 2, compiled by Sturge, V. Walworth and A. Shepp, Neblette (1969). The photosensitive material used contains a light-insensitive silver source (e.g., organic silver salt) capable of reduction, a photocatalyst (e.g., silver halide) in a catalytic activity amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This photosensitive material is stable at room temperature. However, when it is heated at a high temperature (e.g., 80.degree. C. or higher) after the exposure, silver is produced through an oxidation-reduction reaction between the silver source (which functions as an oxidizing agent) capable of reduction and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the silver salt capable of reduction in the exposure region provides a black image and this presents a contrast to the non-exposure region. Thus, an image is formed.
Such heat developable photosensitive materials of this type have been known previously, but in most of those sensitive materials, the photosensitive layer is formed by coating a coating liquid containing a solvent of an organic solvent such as toluene, methyl ethyl ketone (MEK), methanol, and the like. Use of such organic solvents as a solvent not only adversely affects human bodies during manufacturing processes but also is disadvantageous in term of costs for recycling the solvents and others.
To cope with this, a method has been considered in which a photosensitive layer (hereinafter referred also to as "aqueous photosensitive layer") is formed using a coating liquid of a water solvent not having the above problem. For example, Japanese Unexamined Patent Publication [KOKAI] (hereinafter referred simply to as "JP-A") Showa Nos. 49-52,626 and 53-116,144, and the like set forth an example that gelatin is used as a binder. Also, JP-A-50-151,138 sets forth an example that a poly vinyl alcohol is used as a binder.
In JP-A-60-28,737, an example that a gelatin and a polyvinyl alcohol are used together is described. In addition, as another example other than the above examples, JP-A-58-28,737 sets forth an example of a photosensitive layer that a water-soluble polyvinyl acetal is used as a binder.
Such a binder surely allows to form the photosensitive layer in use of a coating liquid with a water solvent, and brings advantages in terms of environments and costs.
However, if the hydrophilic polymer such as gelatin, polyvinyl alcohol, water-soluble polyvinyl acetal, and so on is used as the binder of the protection layer, adherence between the image forming layer and the protection layer on the image forming layer becomes very poor after the heat development, thereby raising a problem that the protection layer easily falls off. When a polymer latex having a high hydrophobic nature is used for the binder of the protection layer, the image forming layer shows some adherence after heat development but the heat development causes the added compounds to be deposited outside the layer, thereby raising a problem that white powdery stains (hereinafter, referred to as "white powders") occur.
It is an object of the invention to provide a heat developable image recording material for photomechanical processes, particularly for scanners or image setters, with improved adherence between the image forming layer and the protection layer formed on the image forming layer after the heat development and improved feature against occurrences of the white powders. Furthermore, it is another object of the invention to provide a heat developable image recording material with improvements in film strength adequate to prevent contact failures and with good dimentional stability.